Having completed the amino acid sequence of dogfish M4 lactate dehydrogenase (LDH) and with the crystal structure known, it becomes meaningful to compare structural homologies which may exist in other lactate dehydrogenases. In addition since several unique forms of LDH are known to occur naturally, this enzyme provides an excellent model for investigating the evolution of both enzyme specificity and subunit interaction at the level of the primary structure. A technique has been devised for rapidly comparing the two major active site regions of the molecule. These two regions include the "loop" which shows major conformational changes following coenzyme binding, the essential thiol group, and one of the two residues which are directly involved in substrate binding. Preliminary studies show extensive homologies in these regions for various isoenzyme forms of LDH, including chicken H4, rabbit M4, and beef H4 LDH. On the other hand, lobster tail LDH has unique enzymatic properties, and this difference is indeed reflected in a comparison of these active site regions. Lobster tail LDH shows sigmoidal kinetics and these sigmoidal kinetics can be correlated to the aggregation state of the enzyme in which the dimer and tetramer are in equilibrium. A comparison of the complete primary structure of lobster tail LDH with dogfish M4 LDH should be an excellent model for investigating the evolution of enzyme specificity and subunit interactions. We would like to continue these preliminary investigations and determine the complete amino acid sequence of lobster tail LDH. In contrast to vertebrate LDH's, another class of lactate dehydrogenases exist which are specific for D-lactate rather than L-lactate. These enzymes are found in mollusks, arachnids, and in several bacteria. These LDH's are also dimers rather than tetramers. We would also like to purify a lactate dehydrogenase which is specific for D-lactate and investigate its primary structure in an attempt to correlate these known changes in substrate specificity with differences in the amino acid sequence. An effort will also be made to crystallize the lobster tail LDH and the LDH specific for D-lactate so that the crystal structures of these two enzymes can be investigated.